Currently, since demands for portable electronic products such as laptop computers and mobile phones are rapidly increased and demands for electric carts, electric wheelchairs, electric bikes, etc. are also increased, research is being actively conducted on repeatedly rechargeable high-performance batteries. In addition, due to gradual exhaustion of carbon energy and increased attention to the environment, global demands for hybrid electric vehicles (HEVs) and electric vehicles (EVs) are increased. As such, more attention and research are focused on a battery which is a major part of a HEV or EV, and development of a quick charging technology capable of charging the battery at a high speed is urgently required. In particular, quick charging is a very significant performance factor for an EV having no extra energy source.
A process for charging the battery includes supplying a current to the battery to accumulate charges and energy therein, and requires careful control. In general, an excessive charging rate (C-rate) or charging voltage can permanently deteriorate the performance of the battery and ultimately cause a complete failure or a sudden failure such as explosion or leakage of a corrosive chemical material.
A conventional battery charging method includes a constant current (CC) charging method for charging a battery with a constant current from beginning to end, a constant voltage (CV) charging method for charging a battery with a constant voltage from beginning to end, and a constant current-constant voltage (CC-CV) charging method for charging a battery with a constant current at the beginning and with a constant voltage later.
In the CC charging method, at the beginning, a voltage difference is large and thus a high current flows. Although a high charging current is desirable in terms of quick charging, if a battery is continuously charged with a high current, charging efficiency may be reduced and the lifetime of the battery may be influenced. Furthermore, in the CC charging method, since the current flowing at the beginning of charging continuously flows through the battery after charging is completed, lithium (Li)-plating occurs due to characteristics of Li ions and thus a safety problem of losing an overcharge control function is caused. Therefore, the battery should be detached from a charger immediately after the battery is fully charged. In the CV charging method, when a battery is fully charged, a terminal voltage is greatly changed due to a variation in temperature and heating of the battery and thus a constant voltage value may not be easily set in advance. Furthermore, since the battery is generally charged with about 15.5 to 16V for 20 to 24 hours, a charging time is long.
The most commonly used method is the CC-CV charging method in which a battery is charged with a constant current when the battery is discharged a lot, and then is charged with a constant voltage when the battery is almost fully charged, thereby preventing overcharge. When “C” denotes a battery capacity in a charge unit (often denoted by Q) A·h, a current in amperes is selected as a fraction (or multiplier) of C. A battery is generally charged at up to 1C. For example, a Li battery having a capacity of 700 mAh is fully charged after about 1 hour and 30 minutes. However, in this charging method, the battery should be charged in a condition suitable for the charging capacity of a charger, at a well-ventilated place, and at a room temperature of about 25° C.
The CC charging method is the most advantageous for quick charging. However, when the battery is quickly charged at a high charging current density, Li is not intercalated into but is deposited on a negative electrode and thus Li-plating occurs. Furthermore, the deposited Li may cause problems such as side reaction with an electrolyte and a change in kinetic balance of the battery, and may ultimately cause deterioration of the battery. Therefore, a technology capable of quickly charging a battery without causing Li-plating is required.